The present invention is directed to a method for producing a magnetic bias field in a bubble domain memory device. The method comprises coupling a magnetic element with a permanent magnetic. The permanent magnet is formed of a rare earth metal-alloy.
Several classes of materials are known hitherto for the magnetic bubble domain memory films formed on the surface of a substrate such as a single crystal wafer of gadolinium gallium garnet including films of orthoferrites, magnetoplumbites and rare earth-containing garnets as well as amorphous films thereof. Among the above named classes, the materials currently on practical use are the rare earth-iron garnets expressed by the general formula R.sub.3 Fe.sub.5 O.sub.12, in which R is a rare earth element, from the standpoints of high density of integration and high velocity of operation as exemplified by yttrium iron garnet of the formula Y.sub.3 Fe.sub.5 O.sub.12.
A magnetic bubble domain memory device is constructed with the film of such a magnetic bubble domain material formed on a suitable substrate, to which a magnetic bias field is applied in a strength adequate for the formation of magnetic bubbles in the material. The magnetic bias field is usually obtained with a permanent magnet and the performance thereof naturally influences heavily the performance of the magnetic bubble domain memory device. In this connection, there is a requirement that the temperature coefficient of the magnetic bias field produced by the permanent magnet should be in coincidence with or about the same as the temperature coefficient of the magnetic bubble disappearance field H.sub.o of a given magnetic bubble domain material. In this regard, it is the conventional practice to select a permanent magnet having a temperature coefficient as close as possible to the temperature coefficient of the bubble disappearance field H.sub.o of the given magnetic bubble domain material or to modify the composition and/or the method of preparation of the magnetic bubble domain material so that the material has a modified temperature coefficient of the bubble disappearance field H.sub.o in coincidence with or close to the reversible temperature coefficient of the given permanent magnet.
It is known that the temperature coefficient of the bubble disappearance field H.sub.o of the practical magnetic bubble domain materials ranges widely from -0.1%/.degree.C. to -0.6%/.degree.C. so that no sufficient coverage is obtained with conventional permanent magnets. For example, the reversible temperature coefficients of the so-called Alnico magnets mainly composed of aluminum, nickel, cobalt and iron and the ferrite magnets of which the main constituent is barium ferrite are about -0.03%/.degree.C. and about -0.2%/.degree.C., respectively. On the other hand, rare earth metal-containing permanent magnets, mainly composed of a rare earth metal and cobalt, have temperature coefficient of about -0.03%/.degree.C. in the samarium-cobalt based ones and about -0.09%/.degree.C. in the cerium-cobalt based ones. Therefore, these rare earth metal-containing permanent magnets are not satisfactory in practical use as a bias magnet in the magnetic bubble domain memory devices despite their excellent properties as a permanent magnet for general purpose. Accordingly, it has been the most widely practiced way in the magnetic bubble domain memory devices that the temperature coefficient of the bubble disappearance field H.sub.o in the rare earth-iron garnet as the magnetic bubble domain material is modified by the addition of a small amount of a modifier element such as lutetium and the like into the garnet material so that the temperature coefficient of the bubble disappearance field thereof is brought into coincidence as close as possible with the temperature coefficient of the ferrite magnets as the permanent magnet for the bias field although no quite satisfactory results have yet been obtained.